A large number of torque converters are provided with a lock-up device for directly transmitting torque from a front cover to a turbine. The lock-up devices of this type includes a piston, a drive plate, a plurality of torsion springs and a driven plate. The piston is allowed to be frictionally coupled to the front cover. The drive plate is fixed to the piston. The torsion springs are supported by the drive plate. The driven plate is elastically coupled to the piston in a rotational direction by means of the torsion springs. Further, the driven plate is fixed to the turbine.
The piston axially divides the space between the front cover and the turbine. The piston is axially movable by means of hydraulic pressure difference between the axially divided spaces. Torque is transmitted from the front cover to the lock-up device when a friction facing, annularly attached to the radial outer part of the piston, is pressed onto the flat frictional surface of the front cover.
When torque transmission is herein executed through the lock-up device, low stiffness and a wide torsion angle are required for the torsion springs in order to eliminate or attenuate variation in torque to be inputted thereto from an engine. In response, devices have been proposed, for instance, as described in the patent document No. 1. In the device of the patent document No. 1, torsion springs are disposed on the radial inner part and the radial outer part of the lock-up device. Further, an intermediate member couples the radial inner side torsion springs and the radial outer side torsion springs.